What is Variation?
Variation refers to the differences that exist among individuals of the same species. No two people (except identical twins) are exactly alike. Variation arises from two main sources: genes inherited from parents and the environment in which an organism lives.
Two Types of Variation
Variation is divided into two categories according to how the differences are distributed.
- Continuous variation — a smooth range of values between two extremes, with many intermediate forms. Examples: height, body mass and hand span. It is usually controlled by many genes and is strongly influenced by the environment. A graph of the data gives a bell-shaped curve.
- Discontinuous variation — distinct categories with no in-between values. Examples: blood group (A, B, AB, O), the ability to roll the tongue, and gender. It is usually controlled by one or a few genes and is little affected by the environment. A graph gives separate bars.
Key idea
Continuous = a range with intermediates (e.g. height). Discontinuous = clear-cut groups (e.g. blood group). Continuous traits are affected by the environment; discontinuous traits usually are not.
Example
If we measure the heights of all Form 4 students in a school, the values form a continuous range from short to tall. But if we record their blood groups, everyone falls neatly into A, B, AB or O — discontinuous variation.
Why Variation is Important
Variation makes a population able to survive changes in the environment. If conditions change, some individuals with useful features are more likely to survive and reproduce. Variation is therefore the basis of natural selection and evolution.
It is also important to know that some differences are acquired during an organism's lifetime, such as a scar or a suntan. These are not passed to the offspring because they do not change the genes carried in the sex cells. Only variation that is caused by genes can be inherited by the next generation.
Remember
Continuous variation is influenced by both genes and the environment; discontinuous variation is controlled mainly by genes alone.